1. Field of the Invention
The present invention relates to a semiconductor device having circuits composed of thin-film transistors. More particularly, the present invention relates to an electro-optical device represented by a liquid-crystal display unit and also to a structure of electronic machines and equipment equipped with such electro-optical devices. The term “semiconductor device” as used in this specification generally denotes any device which functions by utilizing the semiconductor characteristics and hence it embraces said electro-optical devices and electronic machines and equipment equipped with said electro-optical devices.
2. Description of the Related Arts
Considerable efforts have been directed to the application of thin-film transistors (TFT for short hereinafter) to liquid crystal display units of active matrix type because of their ability to be formed on a transparent glass substrate. TFTs based on polysilicon film have such high mobility that they are expected to give a precision image if their functional circuits are integrated on a single substrate.
A liquid crystal display unit of active matrix type needs as many TFTs as million for pixels alone if it is to produce an image of high resolution. When it is provided with functional circuits, it needs more TFTs. For its stable operation, it requires individual TFTs to work with good stability and reliability.
In a liquid crystal display unit of active matrix type, each pixel is constructed of n-channel type TFT, which is required to meet the characteristics of both on-region and off-region because a gate voltage applied to it ranges from about 15 to 20V. On the other hand, peripheral circuits to drive the pixel are composed mainly of CMOS circuits, which rely greatly on the characteristics of on-region.
Unfortunately, TFTs made of polysilicon film tend to increase in off-current (leakage current). This leads to a decrease in mobility and on-current after prolonged operation. A probable reason for this trouble is an increase in channel electric field which gives rise to hot carriers and deteriorates the characteristic properties.
A well-known way in the field of MOS transistor to prevent the deterioration of characteristic properties due to hot carriers, thereby improving reliability, is to adopt the LDD (Lightly Doped Drain) structure. This structure is characterized by an impurity region of lower concentration which is formed inside the source-drain region. This region is called LDD region. The LDD structure permits the off-current to be reduced more than the ordinary TFT structure.
MOS transistors are constructed such that the LDD region overlaps to some extent with the gate electrode, with a gate insulating film interposed between them. This structure is known as, for example, GOLD (Gate-drain overlapped LDD) and LATID (Large-tilt-angle implanted drain), which are made in different ways. These structures make it possible to reduce the impurity concentration in the LDD region, thereby reliving the electric field and improving the hot carrier resistance.
Attempts are being made to apply the technology of MOS transistor to TFT. For example, GOLD structure with side wall of silicon is reported in “M. Hatano, H. Akimoto, and T. Sakai, IEDM97 TECHNICAL DIGEST, p. 523–526, 1997”.
The structure reported in this paper presents difficulties in leaving the gate insulating film highly selectively in the anisotropic etching of film for the side wall. This leads to variation in characteristic properties.
In addition, the structure reported in this paper has a larger off-current (the current that flows when TFT is in off state) than the ordinary LDD structure. Therefore, when TFTs of the reported structure are used as switching elements to drive pixel electrodes, they consume more electric power and display anomalous images due to large off-current. The increase in off-current is due to the fact that an inversion layer is formed in the LDD region which overlaps with the gate electrode and this inversion layer functions as the hole passage.